PROJECT SUMMARY: Intestinal Paneth cells and some myeloid cells secrete lysozyme, a glycoside hydrolase that catalytically cleaves bacterial cell-wall as well as exerts bactericidal activity through a non-enzymatic domain. This feature uniquely distinguishes lysozyme from other antimicrobial peptides. Elevated levels of intestinal lysozyme and its ectopic production by colonic mucosa are observed in clinically active intestinal inflammation conditions. Recent studies described defective lysozyme packaging and secretion in Paneth cells of Crohn?s disease (CD) patients. Unfortunately, existing literature suggested both a colitis-promoting and a colitis-protective role of lysozyme. The human LYZ gene is located in the vicinity of an Ulcerative Colitis (UC) risk locus, and patients with LYZ mutations exhibit IBD symptoms. Despite of these strong associations of intestinal lysozyme with UC and CD, surprisingly little is known about its exact physiological functions in regulating gut microbiota and mucosal immune response. This knowledge gap exists due to a lack of models for direct study of lysozyme in vivo function. This gap prevents us from fully understanding the clinical implication of lysozyme in the progression and management of distinct IBD forms. We have developed several mouse models to facilitate functional investigation of intestinal lysozyme under its deficient or over production conditions. Preliminary data suggest that lysozyme deficiency leads to a dysbiosis with an expansion in certain IBD-related bacterial species. When barrier function is acutely compromised, there is an enhanced translocation of opportunistic pathobionts and exacerbated colitis in these mice. Further analyses uncovered lysozyme sensitive microbes, an unexpected impact on epithelial cell differentiation and composition, as well as an influence on the mucosal immune response. This innovative project will investigate how lysozyme defects affect the colonization of IBD-related bacteria, how lysozyme-sensitive species modulate gut mucosa, and how lysozyme and bacterial interaction influences the progression and resolution of different experimental IBDs with distinct inflammatory profiles. The central hypothesis is that intestinal lysozyme modulates gut microbial community and its impact on mucosal immune responses to regulate inflammatory tone and the susceptibility of IBD. This hypothesis is formulated on the basis of preliminary data and existing literature. Two highly cohesive aims will first delineate the impact of lysozyme on gut microbiota, the regulation of mucosal homeostasis and inflammation by the lysozyme-sensitive microbiota, and will then examine the contribution of lysozyme defects to distinct forms of experimental colitis of different immunological profiles, with a particular effort devoted to understanding the role of type 2 immune signaling in these processes. This MPI project is constructed on complementary and indispensable expertise of the two PIs in experimental colitis, mucosal immunology, gnotobiotics study, genetics, and intestinal epithelial biology. The project utilizes rigorous genetic approaches and is expected to produce unique and disease-relevant insights into intestinal lysozyme and its impact on microbiota, mucosal immunology, and IBDs.